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Thread: Fuel Economy Question

  1. #1
    Regular Member cdms811's Avatar
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    Default Fuel Economy Question

    At Which Speed does a 2.2 Direct achieve its best fuel economy?

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    Regular Member poves's Avatar
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    I would have thought 56 mph on the motorway in top gear .
    I can average 45 mpg at that speed in a 3.2 v6 , not that I drive that speed , it was just a test : )

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    Not sure if this equates to max fuel economy, but an engine is at it's most efficient at the point where it's developing its maximum torque, hence why CVT transmissions keep the engine at quite high revs.

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    Regular Member Ste's Avatar
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    Quote Originally Posted by Thomas1 View Post
    Not sure if this equates to max fuel economy, but an engine is at it's most efficient at the point where it's developing its maximum torque
    Categorically NOT true.

    If you look at engine brake specific fuel consumption, optimum operating condition for the engine will be just below peak torque - ie at around that engine speed but at about 90% load.

    How this translates into optimum vehicle speed for fuel consumption totally depends on gearing, aero's, rolling resistance, transmission losses etc etc etc.

    Optimum cruising speed for fuel consumption will depend on a lot of factors, and will change for vehicle models, engine characteristics, drive lines, weights wheels, tyres, roof racks, etc etc.


    So, basically, it is not as simple as looking at the engine in isolation.


    The only way to accurately define it would be to plot mpg against vehicle speed, although this would be limited even so. The road surface and inclination would play a huge factor in it too.

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    Quote Originally Posted by Ste View Post
    Categorically NOT true.

    If you look at engine brake specific fuel consumption, optimum operating condition for the engine will be just below peak torque - ie at around that engine speed but at about 90% load.

    How this translates into optimum vehicle speed for fuel consumption totally depends on gearing, aero's, rolling resistance, transmission losses etc etc etc.

    Optimum cruising speed for fuel consumption will depend on a lot of factors, and will change for vehicle models, engine characteristics, drive lines, weights wheels, tyres, roof racks, etc etc.


    So, basically, it is not as simple as looking at the engine in isolation.


    The only way to accurately define it would be to plot mpg against vehicle speed, although this would be limited even so. The road surface and inclination would play a huge factor in it too.
    What is "categorically not true"? - that engines are at their most efficient at the point they achieve maximum torque? You haven't disproved that at all in your points above.

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    Some bloke somewhere with a laptop and too much time on his hands reckons I'm right:

    http://en.wikipedia.org/wiki/Engine_efficiency

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    Regular Member Ste's Avatar
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    Vehicle : Jaguar XF 3.0D V6

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    We are not talking about thermal efficiency, we are talking about fuel consumption. Just because you are reaching peak Thermal, doesn't mean you are achieving optimum fuel efficiency.

    And peak engine efficiency doesn't mean best fuel consumption for vehicles cruising.

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    Efficiency and economy aren't the same thing.

    A 2.8T engine flat out in 2nd might be quite efficient at turning large amounts of petrol into a change in speed, but it won't be economical.

    Economy = minimum use of petrol for a given journey
    Efficiency = how good an engine is at converting energy stored in petrol into work

    Having said that, the most efficient engines will generally give the best economy.

    What was my point again?

    dv.

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    Quote Originally Posted by VXR_DV View Post
    Efficiency and economy aren't the same thing.

    I think is your (and my) point here

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    Also from wiki: Fuel Efficiency

    This ratio is based on a car's total properties, including its engine properties, its body drag, weight, and rolling resistance (friction), and as such may vary substantially from the profile of the engine alone. While the thermal efficiency of petroleum engines has improved in recent decades, this does not necessarily translate into fuel economy of cars, as people in developed countries tend to buy bigger and heavier cars (i.e. SUVs will get less range per unit fuel than an economy car).

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